Audiometers, particularly audiometers for use on small children or babies, where frequent frequency changes are used to retain the subject's attention, have the additional requirement that switching between frequencies, or between ON-OFF, must be soft, that is, must be such that switching clicks or thumps upon change in frequency, or volume of tone are avoided. Switching clicks are not only annoying to the test subject, but additionally interfere with proper testing. Such switching clicks usually are formed by a group of frequencies of an extended band of frequencies. The American National Standard Institute (ANSI) has developed a specific standard for audiometers which, among others, requires switching without clicks and free from higher harmonics. This condition, by and itself, can be met by raising the level of the tone signal comparatively slowly upon first connecting the tone signal and then, likewise, dropping the level slowly upon disconnection. This solution is not entirely satisfactory, however, since it is in conflict with a further standard of the ANSI which requires short switching times, that is, a relatively steep rise in level, and steep decay or drop in level, upon turn-off. The difference in level, that is, the suppression of sound when the sound is OFF, should be at least 60 dB.
The problem of suppression of switching clicks or thumps has particular significance if the tone signal is to be connected ON and OFF at a high rate so that, for example when testing babies, a high degree of attention is insured.